Typical hot runner valve gated nozzles in plastic molding systems use a single central melt channel to convey the resin from a supply channel in a hot runner manifold to the mold cavity gate. A valve stem is generally employed which is a small diameter rod that is positioned centrally in the nozzle's melt channel and reciprocates to control the opening and closing of the valve gate. Tne resin generally flows in the nozzle channel in a different direction than the resin flow in the hot runner, e.g., the resin may flow in the nozzle channel along an axis perpendicular to the flow in the hot runner. This results in a change of direction of resin from the hot runner to the nozzle and also in the impingement of the resin on the valve stem which factors can create flow characteristics and dead spots that cause undesirable thermal degradation of the resin.
U.S. Pat. No. 4,705,473 to Schmidt attempts to solve this problem by splitting the incoming resin flow to the nozzle. An intermediate element between the manifold and the nozzle contains the single incoming channel and two curved channels extend from the incoming channel and connect to the central nozzle channel on opposed sides. Thus, the melt flow is split in two and is recombined by flowing down opposed sides of the valve stem.
European Patent Application 0,405,007 to Gellert has a similar double channel arrangement that is cast into a single valve member bushing housed inside the hot runner manifold. Again, the melt flow is split into two channels which curve around the central channel to feed into it from opposed sides.
This approach has the disadvantage of splitting the main flow into two distinct and separate paths. At the splitting point the resin is subjected to accelerated shear heating since it must divide into two streams. A second disadvantage is that when the two melt streams recombine in the upper portion of the nozzle channel, they will not completely mix. Their respective melt boundaries may create a "melt front witness" which is visible in the finished part. A third disadvantage of this approach is that to manufacture bushings having two curved channels within them is costly and time consuming. Typically, a "lost-wax" casting approach must be taken in order to develop the smooth channels.
It is therefore a principal object of the present invention to provide a plastic molding system which overcomes the foregoing disadvantages and conveniently conveys resin melt from a hot runner channel to a nozzle channel, even with a change of direction in the resin flow.
It is a still further object of the present invention to provide a system as aforesaid which does not split the resin flow into two channels while conveying the resin as aforesaid.
Further objects and advantages of the present invention will appear hereinbelow.